This invention relates to a method and apparatus for the production of cluster ions. In a preferred embodiment, it relates to a method and apparatus for the generation of large cluster ions with narrow mass distributions. In a further preferred embodiment, it relates to the generation of hydrogen or isotropic hydrogen cluster ions. As the term is used throughout this application, "cluster ions" means an aggregation of atoms or molecules ranging in size from several tens of particles to in excess of several hundred thousand particles and carrying one or more units of electrical charge.
Since cluster ions are charged, they may be accelerated by conventional means. Such accelerated cluster ions are useful in the investigation of the interchange of energy between the electronic and nuclear structures of atoms, and in the investigation of condensed states of matter. They may also prove useful in the production of colloids and the treatment of surfaces. Cluster ions of the type prepared by the present invention may also be useful in doping solid material for potential use in semi-conductors and solid state devices. Isotopic hydrogen cluster ions could be used to produce an intense cluster ion beam for heating plasmas for the ignition of controlled thermonuclear reactions. Isotopic hydrogen cluster ions may have particular use in the generation of neutral beams for use in magnetic confinement fusion apparatus and in other possible methods for the production of fusion energy. Light element-hydrogen cluster ions in which clusters containing mixtures of lithium or boron, for example, may also have valuable application in fusion systems.
Prior to the present invention, neutral clusters were formed by the expansion of gases, usually at room temperature, through supersonic nozzles with the generation of cluster ions through subsequent electron impact ionization. Typically, the gases would flow through a small nozzle from a pressure of about 1 atmosphere to a pressure of about 10.sup.-3 atmosphere. As the gas expanded and cooled small neutral clusters would form. These clusters would be aerodynamically accelerated to speeds greater than sound, hence the term "supersonic". These clusters would then pass through a further series of nozzles, typically referred to as "skimmers" and "collimators" into regions of still lower pressure, ultimately forming a beam of clusters which would then be charged by electron impact ionization, forming cluster ions by secondary emission of electrons.
Gspann, et al. ("Cluster Beams of Hydrogen and Nitrogen Analyzed by Time-of-Flight Mass Spectrometry", Journal of Chemical Physics, Volume 59, pages 4726-4734, November 1973) disclosed the production of neutral clusters of hydrogen and nitrogen from condensing supersonic nozzle flows. The resulting clusters were later ionized by electron bombardment.
The basic method described above has a number of major disadvantages. For the clusters to be most useful, they should be large cluster ions with a narrow mass distribution. Such cluster ions, it has been found, are best produced by forming the clusters at around the boiling point of the entity to be clustered. This usually requires operation at greatly decreased temperatures; for example for hydrogen around 18.degree. K., and for nitrogen and argon around 77.degree. K. Disadvantages resulted from the fact that in procedures such as that described by Gspann, et al., the clusters that were produced were neutral and subsequently had to be ionized. These neutral clusters are only weakly bound and thus they are subject to loss by re-evaporation and they are easily fragmented by electron impact while being charged. Further, the heat associated with producing the impact electrons will tend to re-evaporate the cluster ions, particularly as high electron impact current densities are used to produce high current cluster beams.
These disadvantages of the prior art are overcome by the present invention which provides a method for the generation of large cluster ions by the expansion of a purified ionized gas cooled to about or below its boiling point through a supersonic nozzle.
It is also known to ionize gases and to sample such ionized gases through an aperture opening into a region maintained at a low pressure. Searcy, et al. ("Clustering of Water on Hydrated Protons in a Supersonic Free Jet Expansion", Journal of Chemical Physics, Volume 61, pages 5282-5288, Dec. 15, 1974) made small clusters of water containing up to 28 water molecules and a single proton using a corona discharge to produce ions in gas at room temperature which was subsequently expanded in a supersonic nozzle. The work by Searcy, et al. was not done at low temperatures. Low temperature expansion of plasma requires the development of an ion source free of condensing insulating impurities that inhibit extraction of such ions from the cooled supersonic nozzle.
Devices known as Atmospheric Pressure Ionizers are also intended for use in conjunction with mass spectrometers. Such devices operated at room temperature and above are described by Dzidic, et al., "Comparison of Positive Ions Formed in Nickel-63 and Corona Discharge Ion Sources Using Nitrogen, Argon, Isobutane, Ammonia and Nitric Oxide as Reagents in Atmospheric Pressure Ionization Mass Spectrometry" Analytical Chemistry, Vol 48, No. 12, October 1976, and by Carroll, et al., "Subpicogram Detection System for Gas Phase Analysis Based Upon Atmospheric Pressure Ionization (API) Mass Spectrometry", Analytical Chemistry, Vol. 46, No. 6, May, 1974.
While some small charged clusters are formed by the Atmospheric Pressure Ionizer described, these clusters are considerably below the size range of "cluster ions" as that term is used in the present application. Further, since the Atmospheric Pressure Ionizer is intended as a source for mass spectrometers, charged clusters constitute a drawback in that they distort or interfere with the measurements made. For this reason the "pinhole" aperture described in the above references is heated to minimize clustering.
Thus, it is an object of the present invention to provide a method and apparatus capable of producing higher currents of cluster ions, over a wide range of initial conditions of temperature and pressure, and particularly to produce large clusters with narrow mass distributions.
It is a further object of the present invention to provide a method and apparatus for producing cluster ions which are charged at the amount of formation and thus more strongly bound and less subject to re-evaporation than cluster ions produced heretofore.
It is a further object of the present invention to provide a method and apparatus for the production of cluster ions where the initial clusters need not be subject to electron impact.
It is a further object of the present invention to produce singly charged, high molecular weight hydrogen clusters with narrow mass distributions.